linux-kernel - Re: [char-misc-next, v4, 5/7] watchdog: mei_wdt: register wd device only if required

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Message-ID: <569D07D9.8020902@roeck-us.net>
Date:	Mon, 18 Jan 2016 07:42:17 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	"Winkler, Tomas" <tomas.winkler@...el.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Wim Van Sebroeck <wim@...ana.be>,
	"Usyskin, Alexander" <alexander.usyskin@...el.com>,
	"linux-watchdog@...r.kernel.org" <linux-watchdog@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [char-misc-next, v4, 5/7] watchdog: mei_wdt: register wd device
 only if required

On 01/18/2016 05:19 AM, Winkler, Tomas wrote:
>
>
>> only
>>>> if required
>>>>
>>>> Hi Tomas,
>>>>
>>>> On Fri, Jan 08, 2016 at 12:49:25AM +0200, Winkler, Tomas wrote:
>>>>> From: Alexander Usyskin <alexander.usyskin@...el.com>
>>>>>
>>>>> For Intel Broadwell and newer platforms, the ME device can inform
>>>>> the host whether the watchdog functionality is activated or not.
>>>>> If the watchdog functionality is not activated then the watchdog interface
>>>>> can be not registered and eliminate unnecessary pings and hence lower the
>>>>> power consumption by avoiding waking up the device.
>>>>> The feature can be deactivated also without reboot
>>>>> in that case the watchdog device should be unregistered at runtime.
>>>>>
>>>>> Signed-off-by: Alexander Usyskin <alexander.usyskin@...el.com>
>>>>> Signed-off-by: Tomas Winkler <tomas.winkler@...el.com>
>>>>> ---
>>>>> V2: rework unregistration
>>>>> V3: rebase; implement unregistraion also at runtime
>>>>> V4: Rebase the code over patchset : "watchdog: Replace driver based
>>>> refcounting"
>>>>>
>>>>>    drivers/watchdog/mei_wdt.c | 196
>>>> ++++++++++++++++++++++++++++++++++++++++++---
>>>>>    1 file changed, 187 insertions(+), 9 deletions(-)
>>>>>
>>>>> diff --git a/drivers/watchdog/mei_wdt.c b/drivers/watchdog/mei_wdt.c
>>>>> index e7e3f144f2b0..85b27fc5d4ec 100644
>>>>> --- a/drivers/watchdog/mei_wdt.c
>>>>> +++ b/drivers/watchdog/mei_wdt.c
>>>>>
>>>> [ ... ]
>>>>
>>>>> +static void mei_wdt_unregister_work(struct work_struct *work)
>>>>> +{
>>>>> +	struct mei_wdt *wdt = container_of(work, struct mei_wdt, unregister);
>>>>> +
>>>>> +	mei_wdt_unregister(wdt);
>>>>> +}
>>>>
>>>> Registration is synchronous, unregistration is asynchronous.
>>>>
>>>> Assuming that is on purpose, I think it warrants an explanation.
>>>>
>>> The unregistration is detected on response from the  ping, which is run under
>> same mutex as unregistration.
>>> Tomas
>>>
>>>
>> And that explains why registration can be synchronous and unregistration
>> has to be asynchronous ?
>
> You need to connect the dots but yes.
> The registration is run from the internal ping request (in probe) or from an internal event (in runtime), so the flow is not already locked by the watchdog mutex.
> Hope it helps.

What I was asking for is a comment such as

"We can not unregister directly because a ping operation (triggered
through the watchdog subsystem) is pending and must be completed first."

Guenter